Jar tools are used to free stuck drill pipe or well tools. They provide a substantial shock to the drill string which is transmitted to the stuck tool or "fish" and which is helpful in dislodging it.
Hydraulic jars are jar tools using hydraulic fluid as the working fluid. In these jars, an example of which is disclosed in U.S. Pat. No. 3,949,821, owned by the applicant, a piston is provided which restricts the flow of hydraulic fluid from one side of the piston to the other when the piston is in its home or rest position. It may, however, be moved from its home position to an enlarged area where the fluid is able to rush by the piston allowing the housing and mandrel to impact thus causing the jar. Following the jar, the driller may compress the tool which causes a bump. While the impact or abutment surfaces on the jar stroke are located inside the housing, the bump surfaces are located on the outside of the housing.
It will be noted that the term "jar" refers to the impact caused when the jar tool is placed under a tension load and, similarly, the term "bump" is the impact caused when the jar tool is placed under a compression load.
This construction, however, suffers disadvantages. Since the bump abutment surfaces are located externally of the housing, they are exposed to mud and drill cuttings which create wear. Further, since the mud and cuttings are pressed between the abutment faces when the bump impact occurs, a small amount may be forced through the seals with great pressure. This "squeeze film effect" can cause seal deterioration and bushing and drive spline wear and galling.